Pretty sure it has to do with this:
schnaba:1497536445 wrote
Hi berend!
Came here via google because I had the very same problem with DokuWiki on Synoloy DSM 6.1. Solved it:
In the Synology admin website go to WebStation --> PHP settings, select your php version (5.6 on my Synology) and activate the extensions "mcrypt" and "openssl". Before the DSM upgrade, mcrypt was not necessary. Now it is, apparently. Everything works as expected again.
The "golden link" for me was this one:
https://github.com/kanboard/kanboard/issues/2060
Good luck!